The present invention relates to a magnetic recording apparatus used, for example, as an auxiliary storage of a computer and a magnetic recording medium for use with the apparatus, and in particular, to a thin-film magnetic recording medium suitable for a magnetic recording apparatus having a high surface recording density of at least five gigabits per square inch.
Due to development of the information-oriented society, the quantity of information daily used is rapidly increasing. Consequently, it is highly required to increase the recording density and capacity of magnetic recording apparatus. The magnetic heads employ an inductive head using a voltage change which appears in response to a change of a magnetic flux with respect to time. These heads are used for the data recording and reading operations. In contrast the heads of this type, there has been increasingly used at a higher speed a composite head including a recording head and a read-back or reproducing head in which the read-back head is a magnetoresistive read-back head having higher efficiency or sensitivity. The magnetoresistive head uses a change in its electric resistance in response to a change in leakage flux from a medium on which data to be read is recorded. Moreover, a giant magnetoresistive (GMR) head having still higher efficiency or sensitivity has been developed to be put to practice, the GMR head using a considerably great change in magnetic resistance (giant magnetoresistive effect or spin valve effect) due to a multilayered structure including a plurality of magnetic layers accumulated with a nonmagnetic layer therebetween. The giant magnetoresistive effect or spin valve effect is an effect in which relative directions of magnetization of the plural magnetic layers arranged with a nonmagnetic layer therebetween are changed by a leakage magnetic field from the medium, which resultantly varies the electric resistance.
The magnetic recording medium practically employed at present includes magnetic layers made of alloys primarily based on cobalt such as Coxe2x80x94Crxe2x80x94Pt, Coxe2x80x94Crxe2x80x94Ta and Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta. These cobalt alloys have a hexagonal structure (hcp structure) in which a c axis is an axis for easy magnetization. Therefore, for an in-plane magnetic recording medium in which its magnetization is reversed in a magnetic film plane for the recording of information, the c axis of the cobalt alloys is set to (11.0) orientation for crystallization in which the c axis takes an in-plane direction. However, the (11.0) orientation is unstable and hence it is generally impossible to produce such a cobalt alloy directly on a substrate. To overcome this difficulty, there has been employed a method advantageously using a good matching property of a Cr(100) plane having a body-centered cubic (bcc) structure with the Co(11.0) plane. Namely, an underlayer film of chromium is first formed with (100) orientation on a substrate and then a magnetic layer of a cobalt alloy is epitaxially grown on the underlayer such that the c axis of the magnetic layer of cobalt alloy is oriented (11.0), namely, has an in-plane direction. To further increase the crystal lattice matching property in a boundary between the cobalt alloy magnetic film and the chromium underlayer, there has been adopted a method to add a second element to chromium to increase lattice spacing of the chromium underlayer. This enhances the (11.0) orientation in the cobalt alloy to resultantly increase coercivity thereof. JP-A-62-257618 and JP-A-63-197018 describe examples of the technology above in which elements such as vanadium and titanium are added to the chromium underlayer. In addition to the increase in coercivity of the recording medium, the decrease in noise is an essential factor for a higher recording density. The magnetoresistive head has quite a high reproducing efficiency and hence is suitable for high-density recording. However, the head is highly efficient not only to reproduced signals from a magnetic recording medium but also highly sensitive to the noise. Therefore, it is required to much more reduce the noise as compared with the recording medium. Japanese Patent No. 2650282 proposes alloy films, for example, a Crxe2x80x94Mo film as an underlayer which increases coercivity and coercivity squareness and which decreases the medium noise. JP-A-10-228621 describes a combination of a Crxe2x80x94Mo underlayer with a Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta magnetic film. JP-A-4-221418 describes a magnetic recording medium in which a cobalt alloy magnetic layer includes at least platinum and boron to increase coercivity for the increase in the recording density. JP-A-9-293227 describes a combination of a Crxe2x80x94Mo underlayer with a Coxe2x80x94Crxe2x80x94Ptxe2x80x94B magnetic layer.
According to JP-A-10-74314, a Co-based nonmagnetic alloy layer is fabricated below the underlayer to decrease the medium noise. In accordance with JP-A-10-143865, chromium and zirconium which are relatively easily oxidized are added to the Co-based nonmagnetic alloy layer such that a surface of the layer is disposed to atmosphere of oxygen to be slightly oxidized so as to further decrease the medium noise in a stable state. JP-A-9-265619 and JP-A-10-214412 describe magnetic recording medium including a Cr-based underlayer in a b.c.c. structure on an alloy film (seed layer) including zirconium and titanium.
On the other hand, JP-A-1-303624 describes a magnetic recording medium having a high recording density. The medium includes two magnetic layers, i.e., a first magnetic layer is a Co-based recording film and a second magnetic layer is a layer which includes cobalt and chromium as primary elements and to which carbon, titanium, zirconium, niobium, and tungsten are added. However, this medium has a coercivity of 64 kA/m (800 oersted). This is less than the value, 160 kA/m (2000 oersted), required for the present invention. JP-A-5-114128 describes a method to lower the medium noise and to increase the recording density in which the medium includes two magnetic films, i.e., a lower layer is a Coxe2x80x94Crxe2x80x94Ta alloy with a lower medium noise and a higher layer is a Coxe2x80x94Crxe2x80x94Pt alloy with high coercivity.
It has been commonly known that the medium noise can be effectively lowered by reducing sizes of crystal grains of the magnetic film and possibly equalizing grain sizes to each other. The technologies above also use this advantageous effect. Such an example is written in pages 5351 to 5353 of J. Appl. Phys., vol. 79 (1996). Namely, the crystal grains become finer by using a Crxe2x80x94Ti alloy underlayer when compared with the prior art employing a Cr underlayer, and the matching of the lattice constant with respect to the Coxe2x80x94Crxe2x80x94Pt alloy is improved to resultantly increase coercivity. It is also known as described in this article that the medium noise is efficiently lowered by increasing the Cr concentration of the Coxe2x80x94Crxe2x80x94Pt magnetic layer. On the other hand, JP-10-143865 describes a medium including a glass substrate. Namely, for example, when the Crxe2x80x94Ti alloy underlayer and the Coxe2x80x94Crxe2x80x94Pt magnetic film are fabricated after slightly oxidizing a surface of the Coxe2x80x94Crxe2x80x94Zr seed layer, the c axis which is an axis for easy magnetization in the h.c.p. structure of the Coxe2x80x94Crxe2x80x94Pt magnetic layer is oriented to be parallel to the film surface plane, i.e., in (11.0) orientation, and the crystal grains becomes finer, leading to a higher signal-to-noise (S/N) ratio. After fabricating a Coxe2x80x94Crxe2x80x94Zr seed layer with composition of 60 at. % Co-30 at. % Cr-10 at. % Zr, a surface of the seed layer is slightly oxidized and an 80 at. % Cr-20 at. % Ti underlayer and a Coxe2x80x94Crxe2x80x94Pt magnetic film are manufactured thereon to obtain a medium. Having producing samples of medium with different values of chromium concentration as 73 at. % Co-19 at. % Cr-8 at. % Pt, 71 at. % Co-21 at. % Cr-8 at. % Pt, and 69 at. % Co-23 at. % Cr-8 at. % Pt, read/write characteristics are evaluated using a magnetic recording disk having a surface recording density of five gigabits per square inch. As a result, the read/write characteristics are improved as the chromium concentration is increased in the Coxe2x80x94Crxe2x80x94Pt magnetic film. For 69 at. % Co-23 at. % Cr-8 at. % Pt, the medium noise takes a minimum value and satisfies the requirement specified. Reduction in read outputs from these medium containing data written in 7090 fr/mm (180 FCI) was examined at about 1000 hours after the data write operation. Results show that the read outputs are kept unchanged for the medium of 73 at. % Co-19 at. % Cr-8 at. % Pt and 71 at. % Co-21 at. % Cr-8 at. % Pt. However, the read outputs are reduced about 5% for the medium of 69 at. % Co-23 at. % Cr-8 at. % Pt. If the rate of reduction in the read outputs is assumed to be fixed with respect to lapse of time, it is expected that the read outputs are lowered about 10% in ten years. This is practically a considerable problem for the magnetic recording apparatus. In pages 1528 to 1533 of IEEE Trans. on Magn., vol. 34 (1998), this problem is discussed as a problem of thermal fluctuation in which the intensity of magnetization recorded on a medium decreases with a lapse of time. The medium including the 69 at. % Co-23 at. % Cr-8 at. % Pt magnetic film has a low S* value of 0.65 and hence a lower recording resolution, which does not satisfy the requirement specified. To examine the problem of thermal fluctuation, the Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta magnetic film and the Crxe2x80x94Mo underlayer described in JP-A-10-228621 are combined with each other to fabricate a sample. This sample shows a low reduction in the read outputs and a favorable characteristic against thermal fluctuation. However, it has been found that since the crystal grains become greater for the Crxe2x80x94Mo underlayer, the medium noise is increased. On a Crxe2x80x94Ti underlayer which is oriented with (200) and which has finer crystal grain sizes, a Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta magnetic film was fabricated. With a Ta concentration equal to or more than about 2 at. %, which is effective to withstand thermal fluctuation, the c axis of the magnetic film disperses in a three-dimensional manner with respect to the film plane and hence the (11.0) plane cannot be epitaxially grown. Consequently, the magnetic recording medium in which a Crxe2x80x94Ti underlayer and a Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta magnetic film are accumulated shows low coercivity and low coercivity squareness, and hence there are obtained only insufficient read/write characteristics.
As an example of the medium structure described in JP-A-9-293227, samples of medium including a combination of a Coxe2x80x94Crxe2x80x94Ptxe2x80x94B magnetic film and a Crxe2x80x94Mo underlayer are fabricated. It has been found that the medium has a low reduction in the read outputs and a favorable characteristic against thermal fluctuation. However, it has been recognized that the medium noise is increased since crystal grains become greater for the Crxe2x80x94Mo underlayer. On a Crxe2x80x94Ti underlayer which is oriented (200) and which has finer crystal grain sizes, a Coxe2x80x94Crxe2x80x94Ptxe2x80x94B magnetic film was fabricated. With a boron concentration equal to or more than about 2 at. %, effective to withstand thermal fluctuation, the c axis of the magnetic film in the h.c.p. structure disperses in a three-dimensional manner with respect to the substrate plane. namely, the (11.0) plane cannot be epitaxially grown. Therefore, the magnetic recording medium in which a Crxe2x80x94Ti underlayer and a Coxe2x80x94Crxe2x80x94Ptxe2x80x94B magnetic film are accumulated shows low coercivity and low coercivity squareness, leading only to insufficient read/write characteristics.
It is therefore an object of the present invention to provide a magnetic recording medium suitable for implementing a magnetic recording apparatus with a surface recording density of at least five gigabits per square inch in which read outputs are only slightly reduced with respect to time even when the medium noise is low, namely, the medium is resistive against thermal fluctuation and has a high recording resolution, thereby removing the problems above.
The object above can be achieved in accordance with the present invention by quasi-epitaxially growing a magnetic layer including a Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta or Coxe2x80x94Crxe2x80x94Ptxe2x80x94B alloy having lower thermal fluctuation on an underlayer including a Crxe2x80x94Ti nonmagnetic alloy of which grain sizes can be reduced. The inventors have found that the condition above can be satisfied by disposing an underlayer including a Crxe2x80x94Ti-based nonmagnetic alloy and an intermediate layer including Coxe2x80x94Crxe2x80x94Pt. On an underlayer of Crxe2x80x94Ti alloy in a b.c.c. structure oriented (200) with respect to a substrate plane, an intermediate layer of a Coxe2x80x94Crxe2x80x94Pt alloy is fabricated to thereby produce a Coxe2x80x94Crxe2x80x94Pt alloy in an h.c.p. structure oriented (11.0). On the intermediate layer, a magnetic layer including a Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta or Coxe2x80x94Crxe2x80x94Ptxe2x80x94B alloy having an h.c.p. structure like the Coxe2x80x94Crxe2x80x94Pt alloy is grown. Through an epitaxial growth, the c axis, i.e., an axis for easy magnetization is oriented (11.0) to be parallel to the film surface plane. The c axis of the magnetic film having the h.c.p. structure is parallel to the film plane, which enhances magnetic anisotropy in the plane and hence increases the coercivity and the coercivity squareness. This advantageously increases the signal-to-noise ratio and the recording resolution. As a result, there is obtained an in-plane magnetic recording medium which is effective to minimize the crystal grain size of the Crxe2x80x94Ti underlayer and which is resistive against thermal fluctuation of the magnetic film including a Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta alloy. The Crxe2x80x94Ti underlayer has a Ti concentration from about 10 at. % to about 26 at. %, favorably, from about 14 at. % to about 24 at. % for the following reason. In this Ti concentration range, the crystal grain size can be easily reduced. In this connection, a Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta alloy includes cobalt as its primary constituent element and further includes at least chromium, platinum, and titanium. In addition to a Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta alloy, there may be used a Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta alloy to which niobium, boron, and titanium are added, the concentration thereof ranging from about one at. % to about three at. %. In this constitution, the crystal grain sizes can be reduced and homogenized, leading to a favorable effect. When boron is added, the grain size can be minimized and the coercivity can be increased, which is quite desirable to increase the recording density. Moreover, a Coxe2x80x94Crxe2x80x94Ptxe2x80x94B alloy is an alloy including cobalt as its primary element and at least chromium, platinum, and boron.
The intermediate layer including the Coxe2x80x94Crxe2x80x94Pt alloy is favorably a ferromagnetic material. Namely, it is then possible to control such magnetic properties as coercivity, coercivity squareness, and residual magnetism. However, when the chromium content is insufficient in the intermediate layer of Coxe2x80x94Crxe2x80x94Pt alloy, the medium noise becomes greater. When the chromium content is excessive in the Coxe2x80x94Crxe2x80x94Pt alloy layer, the coercivity is lowered. Therefore, the chromium concentration is appropriately set to from about 18 at. % to about 24 at. %. The chromium content is more favorably in a range from about 20 at. % to about 24 at. % because the medium noise is remarkably minimized under this condition. To control the coercivity in a range from about 175 kA/m (2.2 kilooersted) to about 287 kA/m (3.5 kilooersted), the platinum concentration is appropriately ranges from about 8 at. % to about 20 at. %. When the tantalum concentration is about 1.5 at. % or less in the Coxe2x80x94Crxe2x80x94Pt intermediate layer, the medium noise is advantageously decreased. This is remarkable especially when a Coxe2x80x94Crxe2x80x94Ptxe2x80x94B alloy is employed as the magnetic film. In this situation, when the tantalum concentration exceeds 1.5 at. %, the (11.0) orientation of the Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta intermediate layer is disturbed, which considerably reduces the coercivity and the coercivity squareness. Therefore, the tantalum concentration is favorably equal to or less than 1.5 at. %. For a high surface recording density of at least ten gigabits per square inch, the chromium concentration in the Coxe2x80x94Crxe2x80x94Pt intermediate layer is favorably set to about 28 at. % or more so that the layer becomes a nonmagnetic film, which is more resistive against thermal demagnetization.
To achieve the object above, it is not necessarily needed to fabricate a two-layer cobalt alloy film in which a Coxe2x80x94Crxe2x80x94Pt intermediate layer is fabricated below a magnetic layer of a Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta or Coxe2x80x94Crxe2x80x94Ptxe2x80x94B alloy. Namely, the advantage can be similarly obtained by employing a magnetic layer of a Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta or Coxe2x80x94Crxe2x80x94Ptxe2x80x94B alloy having composition of a Coxe2x80x94Crxe2x80x94Pt as follows in the proximity of a surface thereof which is brought into contact with the Crxe2x80x94Ti underlayer. The composition includes the tantalum or boron concentration which consecutively increases toward a medium surface along the growing direction thereof. When a magnetic film of which the tantalum or boron concentration has a gradient in a direction of film thickness is used, since the magnetic film includes a reduced number of lattice defects, the crystal grain sizes become more homogenous. This also improves magnetic continuity of the magnetic film in the film growing direction and hence the inversion of magnetization becomes abrupt in the recording operation, which favorably reduces the medium noise.
When a second underlayer which includes chromium as its primary element and which includes at least one of molybdenum and tungsten is fabricated between an underlayer of a nonmagnetic alloy containing chromium and titanium and an intermediate layer of a Coxe2x80x94Crxe2x80x94Pt alloy, there is obtained high coercivity suitable for a higher recording density. This advantage is remarkable when the platinum concentration of the magnetic film of the Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta or Coxe2x80x94Crxe2x80x94Ptxe2x80x94B alloy ranges from about 12 at. % to about 20 at. % and the total concentration of molybdenum and tungsten in the second underlayer ranges from about 16 at. % to 50 at. %. When the Coxe2x80x94Crxe2x80x94Pt intermediate layer is at least about four nanometer (nm) thick, the platinum concentration in the intermediate layer desirably ranges from about 12 at. % to about 20 at. % to increase epitaxy of the second underlayer and the magnetic layer via the intermediate layer.
The magnetic film favorably has magnetic properties as follows. The coercivity measured with a magnetic field applied in the in-plane region is at least 200 kA/m (2.5 kilooersted), product Brxc3x97t between residual magnetic flux density Br and film thickness t measured under the same condition is at least 2.0 Txc2x7nm (20 Gaussxc2x7micron) and at most 10 Txc2x7nm (100 Gaussxc2x7micron), there can be obtained favorable read/write characteristics in a recording density equal to or more than five gigabits per square inch. When the coercivity becomes less than 200 kA/m (2.5 kilooersted), the read outputs are lowered in a high recording density exceeding 12000 fr/mm (300 kilo-flux reversals per inch (kFCI)). When product Brxc3x97t exceeds 10 Txc2x7nm (100 Gaussxc2x7micron), the read outputs are lowered in a high recording density exceeding 12000 fr/mm (300 kFCI). When product Brxc3x97t is less than 2.0 Txc2x7nm (20 Gaussxc2x7micron), the read outputs are minimized in a low recording density. When the coercivity measured with a magnetic field applied in the in-plane region is at least 280 kA/m (3.5 kilooersted) and product Brxc3x97t measured under the same condition is at least 2.0 Txc2x7nm (20 Gaussxc2x7micron) and at most 6.5 Txc2x7nm (65 Gaussxc2x7micron), there are favorably obtained satisfactory read/write characteristics in a recording density equal to or more than 20 gigabits per inch. When the coercivity becomes less than 280 kA/m (3.5 kilooersted), the read outputs are lowered in a high recording density exceeding 16000 fr/mm (400 kFCI). When Brxc3x97t exceeds 6.5 Txc2x7nm (65 Gaussxc2x7micron), the read outputs are decreased in a high recording density exceeding 16000 fr/mm (400 kFCI). When Brxc3x97t is less than 2.0 Txc2x7nm (20 Gaussxc2x7micron), the read outputs are disadvantageously lowered in a low recording density.
When a film of a material including carbon as its primary element is fabricated with a width ranging from about 3 nm to about 12 nm on the magnetic layer and a lubricant layer of an adsorptive material such as perfluoroalkyl-polyether is disposed on the film with a thickness of about 1 nm to about 3 nm, there is obtained a reliable magnetic recording medium applicable to a high density recording operation. By using a substrate of an aluminum alloy plated with Nixe2x80x94P, there can also be produced a medium having a reduced medium noise and being more resistive against thermal fluctuation.
In a magnetic recording disk apparatus including the magnetic recording medium above, a driving section to drive the medium in a recording direction, a magnetic head including a recording section and a read-back section, means for relatively moving the head relative to the medium, and read/write signal processing means for inputting a signal to the head and for reproducing a signal from the head, when the read-back section of the magnetic head includes a plurality of conductive magnetic layers of which resistance remarkably changes when a direction of magnetization of each conductive magnetic layer is relatively changed due to an external magnetic field and a magnetoresistive sensor disposed between the conductive magnetic layers, the sensor including a conductive nonmagnetic layer, i.e., the head being configured in a so-called giant magnetoresistive (GMR) head, there can be obtained a signal intensity for a high recording density and hence a reliable magnetic recording disk apparatus having a recording density of at least five gigabits per square inch.
When the magnetic recording medium of the present invention is used in a magnetic recording disk apparatus, it is desirable that the magnetoresistive head includes a magnetoresistive sensor section fabricated between two shield layers which are made of a soft magnetic substance and which are apart from each other about 0.12 micrometers to about 0.2 micrometers. When the gap between the shield layers exceeds 0.2 micrometers, the read output becomes insufficient in a maximum linear recording density exceeding 8700 fr/mm (220 kFCI). When the gap is less than 0.12 micrometers, it is difficult to appropriately retain insulation between the magnetoresistive sensor and each of the shield layers.